Source of hire is usually recorded in the applicant tracking system when someone applies, either automatically from a tracking link or by asking the candidate. Reports then show how many hires each channel produced over a period. A team might find, for example, that one job board brings many applications but few hires, while employee referrals bring fewer applicants who are more often offered the job.
Counting hires alone can mislead. A fuller picture combines source with cost per hire, time to hire, retention and performance, so the team knows which channels bring people who stay. Data quality matters too: inconsistent tagging, or every hire marked as 'careers site', makes the report useless. Reviewing sources by role type, rather than across the whole business, usually gives the most actionable view.